The present invention is directed to electronic packaging and, more particularly, is directed to a method to produce pedestal features in constrained sintered substrates. Applications for pedestal structures include providing a raised contact surface for probing wafers for burn-in and test.
Raised ceramic interconnect platforms, or pedestals, have been earlier demonstrated in free-sintered glass bonded alumina ceramic to produce pedestal substrates for electrically testing integrated devices while on a wafer. Free sintered substrates are not sintered, or fired, under a load or a constraining force. Prior methods to produce pedestal substrates use a green laminate milling process. In this method an embedded polymer sheet acts as a stop indicator and separation aid to remove surrounding unneeded ceramic features, while still in the xe2x80x9cgreenxe2x80x9d or unfired state, prior to sintering.
There are unique problems with creating pedestal features in substrates sintered under a constraining load, such as glass ceramic (GC) substrates for example. These substrates need to be sintered under a load to provide consistent control of surface (XY) features, typically because there is a mismatch in the sintering of the metal and ceramic. In these substrates there is often no flowing glass like in alumina free sintered substrates to help match densification. If the same techniques that are currently used for free sintered substrates were applied to substrates sintered under a load the pedestal portion of the substrate would collapse into the base portion. Another problem is that the features within and on the base portion would be highly distorted since there would be no constraining force on this surface. Therefore there is a need to produce a substrate which will have the desired pedestal or stepped features when processed in a constrained xe2x80x9cZ shrinkage onlyxe2x80x9d sintering environment.
There are a number of solutions proposed by others for forming elevated or stepped features in sintered substrates. Gauci et al. U.S. Pat. No. 5,478,420, the disclosure of which is incorporated by reference herein, describes a process for making cavities in ceramic substrates using a hard insert to maintain the shape of cutouts made in unfired ceramic greensheets during lamination. This disclosure describes a xe2x80x9cfree sinteredxe2x80x9d or unconstrained sintering process. Although the disclosure could be used for alternatively forming pedestals in a xe2x80x9creversexe2x80x9d cavity shape, during a constrained sintering process the cavities would tend to close up and distort under the applied sintering load which is used to prevent distortion of the ceramic surface features. This is in contrast to the present invention which applies to a constrained sintering process.
Balents et al. U.S. Pat. No. 6,033,764, the disclosure of which is incorporated by reference herein, describes a process for producing a bumped substrate assembly. The disclosure describes the build up of circuitry onto a previously sintered ceramic surface using processes such as plating, photo resist coating, surface oxidation and etching. This is in contrast to the present invention which discloses a method to sinter the entire metallization structure in one step.
Fasano et al. U.S. Pat. No. 6,139,666, the disclosure of which is incorporated by reference herein, describes the use of non-sintering contact sheets to control the exterior surface features of constrained sintered ceramic substrates. Although these methods and materials are used in the present invention to produce pedestal features, the prior reference does not disclose how to produce raised features that contain circuitry. Rather, the reference discloses the control of surface feature planar topography and the influence of the type of contact sheet used.
Fasano et al. U.S. Pat. Nos. 5,645,673 and 5,846,361, the disclosure of which is incorporated by reference herein, disclose the formation of a raised central pedestal through the use of negative pattern barrier layers. These disclosures do not address the requirement of a constraining force to control surface XY shrinkage that exist in some applications, such as glass ceramic substrates.
Shigemi et al. WO 00/26957, the disclosure of which is incorporated by reference herein, describes a process to form cavities in constrained sintered ceramic substrates by removing overlying ceramic with a laser. This method is expensive and has a potential problem in that it can lead to cracking defects along the edges that can compromise the strength of the ceramic. The present invention removes the unneeded ceramic portions using a grinding process which is more accurate and produces a smooth surface that mitigates crack sources. The present invention also discloses a process to make the pedestal using a green milling, pre-sinter, process that is not disclosed in the reference.
Notwithstanding the prior art solutions to the problem, there remains a need for a method to produce pedestal features in constrained sintered substrates.
Accordingly, it is a purpose of the present invention to have a process to produce raised ceramic interconnect platforms or pedestals in ceramic substrates that are sintered using a constraining force to improve dimensional control.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.
The purposes and advantages of the present invention have been achieved by providing, according to a first aspect of the invention, a method to produce a raised pedestal on a ceramic substrate fired under a constraining force comprising the steps of:
forming an opening in a non-sintering contact sheet, this opening corresponding to a cross-sectional dimension or surface area of the desired pedestal and having a perimeter;
providing a first set of stacked greensheets;
placing the non-sintering contact sheet with opening on top of the first set of greensheets;
stacking a second set of greensheets on top of the non-sintering contact sheet with opening to form a completed layer stack;
placing additional non-sintering contact sheets, without openings, on the top and bottom of the completed layer stack;
firing or sintering the completed layer stack under a constraining load to form a ceramic substrate;
removing the additional non-sintering contact sheets;
cutting channels in the ceramic substrate corresponding in position to the perimeter of the opening in the non-sintering contact sheet, the depth of the channels being sufficient to contact the patterned non-sintering contact sheet and selectively removing the ceramic material above the non-sintering contact sheet to form a raised pedestal in the ceramic substrate.
According to a second aspect of the invention, there is provided another method to produce a raised pedestal on a ceramic substrate fired under a constraining force comprising the steps of:
forming an opening in a non-sintering contact sheet, the opening corresponding to a cross-sectional dimension or surface area of the desired pedestal and having a perimeter;
stacking a first set of greensheets;
placing the non-sintering contact sheet with opening on top of the first set of greensheets;
stacking a second set of greensheets on top of the non-sintering contact sheet with opening to form a completed layer stack;
placing additional non-sintering contact sheets, without openings, on the top and bottom of the completed layer stack;
cutting channels in the completed layer stack corresponding in position to the perimeter of the opening in the non-sintering contact sheet, the depth of the channels being sufficient to contact the patterned non-sintering contact sheet;
firing or sintering the completed layer stack under a constraining load to form a ceramic substrate;
removing the additional non-sintering contact sheets and selectively removing the ceramic material above the non-sintering contact sheet to form a raised pedestal on the ceramic substrate.
According to a third aspect of the invention, there is provided another method to produce a raised pedestal on a ceramic substrate fired under a constraining force comprising the steps of:
forming an opening in a non-sintering contact sheet, the opening corresponding to a cross-sectional dimension of the desired pedestal and having a perimeter;
stacking a first set of greensheets;
placing the non-sintering contact sheet with opening on top of the first set of greensheets;
stacking a second set of greensheets on top of the non-sintering contact sheet with opening to form a completed layer stack;
placing additional non-sintering contact sheets, without openings, on the top and bottom of the completed layer stack;
cutting channels in the completed layer stack corresponding in position to the perimeter of the opening in the non-sintering contact sheet, the depth of the channels being sufficient to contact the patterned non-sintering contact sheet;
filling the channel with a non-sintering material;
firing or sintering the completed layer stack under a constraining load to form a ceramic substrate;
removing the additional non-sintering contact sheets and selectively removing the ceramic material above the non-sintering contact sheet to form a raised pedestal on the ceramic substrate.